1. Field of the Invention
This invention relates to a device to control the temperature of a thermionic reactor over a wide range of operating temperatures. More particularly, this invention relates to a method and device for controlling nucleate boiling heat transfer between a cesium reservoir of a thermionic reactor and forced coolant flow within a thermionic reactor.
2. Description of the Related Art
The efficient operation of thermionic reactors has been an area of investigation. A conventional thermionic reactor converts the thermal energy of admitted electrons in a cesium plasma directly into electrical energy. The efficient operation of thermionic reactors with integral cesium reservoirs is partly dependent upon establishing and maintaining the reservoir temperature at an optimum value. However, it is difficult to maintain the optimum temperature over a wide range of thermal power levels when the coolant temperature is significantly lower than the reservoir temperature.
In the typical thermionic reactor, a significant coolant flow rate is required to limit the coolant temperature increase. Variations in bulk coolant temperature are created as the coolant picks up waste heat from successive convertors in the course of flowing from the inlet to the outlet of the reactor. This temperature increase may cause the convertors to operate at less than optimum temperatures, thus decreasing the overall efficiency of the plant.
To counter this problem, heat pipes have been used which accomplish heat transfer by phase change of the liquid. In a heat pipe, the heat input end of the heat pipe serves as an evaporator, with the heat causing the liquid to change to vapor. The vapor travels through a large open portion of the heat pipe, the center of which usually is devoid of any structure. The opposite end of the heat pipe is a condenser, which is cooled to condense the vapor and remove heat. The condensate is pumped back to the evaporator end via capillary induced flow in a porous structure. Heat pipes are characterized by a capillary induced flow feature.
Another concept is through the use of a material called K-MAX. K-MAX is described in "K-MAX: A Material with Exceptional Heat Transfer Properties", written by N. S. Rasor and J. L. Desplat in the Proceedings of the 24.sup.th Intersociety of Energy Conversion Engineering Conference, held in Washington, D.C., in August, 1989, the disclosure which is hereby incorporated by reference. K-MAX is similar to a heat pipe, in that it utilizes a capillary induced flow to return a condensate to a heated surface. It consists of a solid bi-porous material that employs two phase flow for heat transfer. The material has large interconnected pores, forming vapor transport channels, imbedded in a continuous liquid-soaked porous matrix, with much smaller interconnected pores forming liquid transport channels. The material may be formed into any shape, and can bear mechanical loads. In operation, heat is absorbed on the hot side of K-MAX by vaporization of liquid from small pores. The vapor flows through the large pores, condenses on the cold side, and the liquid returns to the hot side by wicking through the small pores.
The disadvantages of these two techniques is a required use of capillary induced flow, which is often too complex to implement. Thus, there is a need in the art for a heat transfer device which does not require capillary induced flow capability. Further, there is a need in the art for more efficiently maintaining the temperature of a thermionic convertor, regardless of the local bulk coolant temperature.